The present invention relates, in general, to optics measuring and testing and, in particular, to lens or reflective image former testing.
Light is produced by the transition of electrons from higher energy states to lower energy states. The law of conservation of energy is satisfied in these transition processes by the emission of a photon, or quantum of light, whose energy corresponds to the difference in energy of the initial and final energy states of the electron. In nature, photons are created spontaneously at many different frequencies. In Light Amplification by Stimulated Emission of Radiation (laser), a light-wave having a wavelength that corresponds to the difference between a high energy state and a low energy state that strikes an electron in the high energy state will cause the electron to transition to the low energy state and emit a photon with the same direction, phase, polarization and frequency as the incident photon of the light wave. Thus, a traveling light wave of a certain frequency is produced.
A semiconductor ridge laser is useful as an element in a photonic integrated circuit because it emits light horizontally which can be processed by another element which is formed on the horizontal plane of the substrate of the photonic integrated circuit. A ridge laser, which includes an active layer in which a traveling light wave is stimulated, has been produced by many different means. Reflectance modification can be applied on each end, or facet, of the ridge laser to reflect light in a desired phase. The reflectivity of a facet after reflectance modification is a measure of the amount of light that the reflectance modified facet directs back into the laser. An increase in reflectivity causes an increase in the amount of light to be reflected back into, and a lower loss from, a laser. The gain available for laser operation is proportional to the length of the laser cavity. A lower loss allows a laser to operate with lower gain, and hence, a shorter cavity length. A shorter laser is more attractive as an element in a photonic integrated circuit than a longer laser because the shorter laser occupies less space on the integrated circuit and, therefore, more of them may be used per unit area. The functionality of a photonic integrated circuit is proportional to the packing density of the elements of the integrated circuit. Therefore, reflectivity is an important design criteria for lasers. If the reflectivity of a facet after reflectance modification can be measured more accurately, then the parameters of a laser (e.g., length, types of reflectance modification processes used) may be optimized for a desired performance level.
In an article entitled xe2x80x9cHIGH-BRIGHTNESS, HIGH-EFFICIENCY, SINGLE-QUANTUM-WELL LASER DIODE ARRAY,xe2x80x9d by D. F. Welch et al., published by the IEE in the Nov. 5, 1987 issue of Electronics Letters, Vol. 23, No. 23, pp. 1240-1241, an equation is disclosed that relates threshold current density of a laser to the reflectivity of the facets on each end of the laser. The equation is as follows:
Jth=J1+(d/xcex93A)[xcex1+([ln(1/R1R2)]/2L)],
where
Jth is threshold current density,
Jt is transparency current density,
d is quantum-well thickness,
xcex93 is the confinement factor of the laser,
A is a constant equal to 0.043 cm um/A,
xcex1 is internal loss,
R1 is the reflectivity of the front facet,
R2 is the reflectivity of the rear facet, and
L is the cavity length of the laser.
The equation does not disclose the method of the present invention.
U.S. Pat. No. 5,103,106, entitled xe2x80x9cREFLECTIVE OPTICAL INSTRUMENT FOR MEASURING SURFACE REFLECTANCE,xe2x80x9d discloses a device for measuring characteristics of a specimen by projecting a beam onto the specimen at a certain angle and receiving the beam reflected off of the specimen at the same angle by one or more photo-sensors. The present invention does not measure reflectance as does U.S. Pat. No. 5,103,106. U.S. Pat. No. 5,103,106 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,848,088, entitled xe2x80x9cSURFACE EMISSION TYPE SEMICONDUCTOR FOR LASER WITH OPTICAL DETECTOR, METHOD OF MANUFACTURING THEREOF, AND SENSOR USING THE SAME,xe2x80x9d discloses a method of measuring reflectance by shining a light at a particular wavelength at a semiconductor layer and a mirror thereon, detecting the spectrum reflected, and measuring the profile of the light reflected. The present invention does not measure reflectance as does U.S. Pat. No. 5,848,088. U.S. Pat. No. 5,848,088 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,052,191, entitled xe2x80x9cCOATING THICKNESS MEASUREMENT SYSTEM AND METHOD OF MEASURING A COATING THICKNESS,xe2x80x9d discloses a device for and method of measuring the thickness of a coating by shining a light source at a particular angle, collecting the light reflected off of the coating, measuring the intensity of the reflected beam to determine the reflectivity of the coating. The present invention does not measure reflectance as does U.S. Pat. No. 6,052,191. U.S. Pat. No. 6,052,191 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 6,054,868, entitled xe2x80x9cAPPARATUS AND METHOD FOR MEASURING A PROPERTY OF A LAYER IN A MULTILAYERED STRUCTURE,xe2x80x9d discloses a device for and method of measuring reflectance by focusing a heating beam onto a region in which a probe beam travels, modulating the power of the heating beam, measuring the reflected power in the probe beam to determine the reflectivity of the region. The present invention does not measure reflectance as does U.S. Pat. No. 6,054,868. U.S. Pat. No. 6,054,868 is hereby incorporated by reference into the specification of the present invention.
It is an object of the present invention to determine the reflectivity of at least one semiconductor laser facet after the reflectance of the facet has been modified.
It is another object of the present invention to determine the reflectivity of at least one semiconductor laser facet after reflectance modification without having to measure the intensity of a reflected beam but by measuring the threshold current density of a semiconductor laser before and after facet reflectance modification.
The present invention is a method of determining reflectivity of a semiconductor laser facet. The first step of the method is fabricating a first semiconductor laser and a second semiconductor laser. The first laser has a cavity length L1 while the second laser cavity length L2. Manufacturing of the lasers is such that the facet reflectivity R0 is the same for the first and second semiconductor lasers.
The second step of the method is determining what is the facet reflectance R0.
The third step of the method is measuring a threshold current density J1 of the first semiconductor laser.
The fourth step of the method is measuring a threshold current density J2 of the second semiconductor laser.
The fifth step of the method is setting variables u=1, x=1, and y=1 if the reflectance of the first facet of the first semiconductor laser is modified and the second facet of the first semiconductor laser is not modified.
The sixth step of the method is setting variables u=1, x=1, and y=0.5 if the reflectance of the first facet and the second facet of the first semiconductor laser are modified to the same extent.
The seventh step of the method is setting u=1, x=1, and y=1 if the reflectance of the first facet of the first semiconductor laser is not modified and the second facet of the first semiconductor laser is modified.
The eighth step of the method is measuring a threshold current density J3 of the modified first semiconductor laser.
The ninth step of the method is determining the reflectance of the first semiconductor laser as follows:
R1=(u){(R0)Exp[xxe2x88x92(2y[(1/L1)xe2x88x92(1/L2)]L1(J1xe2x88x92J3)/(J1xe2x88x92J2))]},
where Exp[xxe2x88x92(2y[(1/L1)xe2x88x92(1/L2)]L1(J1xe2x88x92J3)/(J1xe2x88x92J2))] is an exponentiation function that makes the expression within the brackets the exponent of the preceding entity.